That being said, mentorship is key. The Scoutmaster is key in this. Perhaps if your troop has some Eagle allumni floating around the edges, get them involved in mentoring your SPL. Mostly however, the relationship is between the Scoutmaster and SPL. Check up on the SPL. Find out what his plans are. Help him expand and clarify his meeting plans/trip plans. Give him gentle nudges during the PLC, but NEVER contradict him in front of the Troop. Nothing undermines his authority more or his standing in front of the other Scouts more than the adult stepping in and correcting him in front of everyone. (Obviously safety issues are an exception here.) The SPL should feel comfortable going to the Scoutmaster when he needs help or has a question, if he doesn't, thats a HUGE problem.
Yours in Scouting,